If you're referring to the line 15 on the 1040, Other taxes, it comes from Schedule 2, line 10 of the 1040.
It could be any of the following:
- Self-employment tax
- Unreported social security and Medicare tax from Forms 4137 or 8919
- Additional tax on IRAs, etc.
- Household employment tax
- Repayment of the first-time Homebuyer Credit
If you're not referring to line 15 of the 1040, please respond in this thread with which line in the 1040 you have a question about,.